1. Field of the Invention
The present invention relates generally to printed circuit boards. More specifically, the invention relates to a multi-layer printed circuit board containing an integrated twisted pair conductor having the characteristics of a transmission line.
2. Description of the Related Art
It has recently been customary to design printed circuit boards with line conductors to transmit signals from one end of the boards to another. For example, U.S. Pat. No. 5,278,524, issued to Mullen, discloses the fabrication of a printed circuit board with a coax-like transmission line. The transmission line includes a narrow conductive strip surrounded by four constant voltage conductors. The conductors are spaced from the conductive strip by a dielectric layer. U.S. Pat. No. 4,310,811, issued to Currie, discloses a printed circuit board with external rework wires. The rework wires are part of a dedicated rework layer. The dedicated rework layer comprises a rework reference layer that is affixed to the printed circuit and an overlying network of wires that are semi-permanently affixed to the rework reference layer for facilitating the reconnection of components or overcoming defects in the buried signal layers. However, although these patents disclose fabricating a printed circuit board with line conductors, neither one discloses the fabrication of a printed circuit board with an integrated twisted pair conductor.
As is well known in the field of transmission lines, a twisted wire pair conductor (i.e., a twisted signal and return wire conductors) provides a simple way of reducing electromagnetic interference in a cable interconnecting two circuits. That is, the twisting of the wire conductors reduces the susceptibility of coupling external signals (e.g., noise signal) with the desired signal. It is also well known that a twisted wire pair conductor allows for the easy removal of external signals collected by a transmission line. For example, due to the signal and return wires being intertwined, any external signal collected by one wire is also collected by the other wire. Thus, a differential receiver can be used, at the receiving end of the twisted wire pair conductor to filter out the signal common in both wires (i.e., the collected external signal). It is further well known that the reduced loop area formed by the signal and return wires of a twisted pair conductor greatly minimizes magnetic coupling on the transmission line.
In the past, whenever it was desired to have two chips connected with a twisted wire pair conductor on a printed circuit board, an external twisted wire pair cable was used. The cable was soldered to the trace conductors on the printed circuit board leading to particular pins of the chips to be connected. This was a labor and time intensive project as the conductor traces on the printed circuit boards are thinly sized and closely spaced together. In addition, the cost of the electrical component system containing the printed circuit board with the interconnected chips was increased as the cost of the twisted wire cable was added to the cost of the overall system.
Moreover, whenever both analog and digital electrical components were used on the same printed circuit board, they were situated such that all the digital components Were at one end of the board and all the analog components at the other end. This configuration was used to minimize the chance of introducing unwanted signals on the analog lines due to the switching of the digital components. Furthermore at times, two printed circuit boards were used to minimize the likelihood of introducing these noise signals in the system, one of the printed circuit board was used for the analog components and the other for the digital components. This arrangement increased the price of the electrical component system incorporating these printed circuit boards as two printed circuit boards were used instead of one.
Thus, there has been a long felt need in the art for a printed circuit board incorporating a twisted pair conductor for providing electrical connection between two electrical components.